1. Field of the Invention
The present invention relates to a servo motor controlling method and, more particularly to a servo motor controlling method or driving a feeding shaft of a machine tool or an arm of an industrial robot.
2. Related Art
In order to improve a movement accuracy of a numerical control (NC) machine tool or an industrial robot, an accuracy of shape of each component needs to be improved, and assembling of mechanisms and adjusting of a control system need to be accurately performed so as to optimally control operations.
For example in a general servo motor control system for a servo motor in a machine tool which performs two-axis arc interpolation feeding movement, a position loop for allowing a deviation between an NC position command and a feedback signal output, from a position detector disposed a servo motor or a moving body such as a table saddle driven by the servo motor to be zero is provided, and a velocity loop for a velocity feedback signal obtained by differentiating a position feedback signal is provided to use an output of a velocity loop gain as a torque control value.
However, in such a servo motor control system, wherein a rotating direction of the servo motor is reversed, the moving direction of the moving body cannot be directly reversed. This is caused from a lost motion or friction in a feed driving mechanism. When such a circular-arc cutting is performed, a quadrant of a circular arc is changed, and an actual moving path deviates out from a commanded path, so that a bulge of path may occur. The phenomenon is referred to as a stick motion or a quadrant protrusion and becomes one of factors of deterioration in accuracy of contour machining which is one of important operations of the machine tool.
The phenomenon is regarded to cause temporary stoppage of a feeding shaft. More specifically, when the moving direction is reversed, a torque command corresponding to a frictional torque needed to be reversed. However, in a guide, a ball bearing, a ball screw or the like, a three-point contact becomes a two-point contact when the moving direction of the servo motor is reversed. Therefore, a delay occurs in the velocity loop, so that the feeding shaft may be temporarily stopped.
In order to correct the error, techniques for correction of torque at the time of reversion are disclosed. Particularly, Japanese Patent Application Laid-Open Publication (JP-A) No. 10-63325 (Patent document) previously filed by the applicant of the present invention describes that the torque is assisted only at the time of moving direction reversion.
In addition to the phenomenon, recent researches show that a friction after reversion is changed in a free zone, that is, a low-friction region of a ball screw (referred to as a ball contact number changing region). If the changing friction is not corrected, the protrusion-shaped machining error remains, or a depressed portion may occur.
For example, since the technique disclosed in JP-A-10-63325 cannot cope with the change in friction caused from the aforementioned free zone, the protrusion-shaped machining error cannot become zero.
As a prior art for coping with a change in friction caused from the free zone, JP-A-7-13631 describes a technique for performing control by determining starting and ending positions of correction based on a moving amount from reversion of the servo motor and providing a velocity command.
However, in the technique disclosed in JP-A-7-13631 (Patent document 2), since the velocity command is corrected, the correction amount is changed according to a passing velocity in the same tool. Therefore, command amounts corresponding to various passing velocities need to be prepared, so that the control becomes complicated.